Wafer level creation of multiple optical elements

ABSTRACT

Integrated multiple optical elements may be formed by bonding substrates containing such optical elements together or by providing optical elements on either side of the wafer substrate. The wafer is subsequently diced to obtain the individual units themselves. The optical elements may be formed lithographically, directly, or using a lithographically generated master to emboss the elements. Alignment features facilitate the efficient production of such integrated multiple optical elements, as well as post creation processing thereof on the wafer level.

CROSS REFERENCES TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority under 35 U.S.C.§120 to U.S. application Ser. No. 08/727,837, filed Sep. 27, 1996,entitled “Integrated Optical Head for Disk Drives and Method of Formingthe Same” and U.S. application Ser. No. 08/917,865, entitled “IntegratedBeam Shaper and Use Thereof” filed Aug. 27, 1997, which are herebyincorporated by reference.

FIELD OF THE INVENTION

[0002] The present invention is directed to integrating multiple opticalelements on a wafer level. In particular, the present invention isdirected to efficient creation of integrated multiple elements.

BACKGROUND OF THE INVENTION

[0003] As the demand for smaller optical components to be used in awider variety of applications increases, the ability to efficientlyproduce such optical elements also increases. In forming such integratedmultiple optical elements at a mass production level, the need foraccurate alignment increases. Further, such alignment is critical whenintegrating more than one optical element.

[0004] Integrated multiple optical elements are multiple opticalelements stacked together along the z-axis, i.e., the direction of thelight propagation. Thus, light travelling along the z-axis passesthrough the multiple elements sequentially. These elements areintegrated such that further alignment of the elements with themselvesis not needed, leaving only the integrated element to be aligned with adesired system, typically containing active elements.

[0005] Many optical systems require multiple optical elements. Suchrequired multiple optical elements include multiple refractive elements,multiple diffractive elements and refractive/diffractive hybridelements. Many of these multiple element systems were formed in the pastby bonding individual elements together or bonding them individually toan alignment structure.

[0006] In bulk or macroscopic optics to be mounted in a machinedalignment structure formed using a mechanical machining tools, thetypical alignment precision that can be achieved is approximately 25-50microns. To achieve a greater level of 15-25 microns, active alignmentis required. Active alignment typically involves turning on a lightsource, e.g., a laser, and sequentially placing each optic down withuncured ultra-violet (UV) adhesive. Then each part is moved, usuallywith a translation stage, until the appropriate response from the laseris achieved. Then the part is held in place and the epoxy is cured withUV light, thereby mounting the element. This is done sequentially foreach element in the system.

[0007] Alignment accuracies of less than 15 microns for individualelements can be achieved using active alignment, but such accuraciesgreatly increase the amount of time spent moving the element. Thisincrease is further compounded when more than one optical element is tobe aligned. Thus, such alignment accuracy is often impractical evenusing active alignment.

[0008] In many newer applications of optics, as in the optical headconfiguration set forth in commonly assigned co-pending application Ser.No. 08/727,837, which is hereby incorporated by reference, and theintegrated beam shaper application noted above, there is a need to makeoptical systems composed of several micro-optical components and inwhich the tolerances needed are much tighter than can be achieved withconventional approaches. In addition to requiring tight tolerances,elements of lower cost are also demanded. The alignment tolerance neededmay be 1 micron to 5 microns, which is very expensive to achieve withconventional methods.

[0009] To achieve greater alignment tolerances, passive alignmenttechniques have been used as set forth in U.S. Pat. No. U.S. Pat. No.5,683,469 to Feldman entitled “Microelectronic Module Having Optical andElectrical Interconnects”. One such passive alignment technique is toplace metal pads on the optics and on the laser and place solder betweenthem and use self-alignment properties to achieve the alignment. Whensolder reflows, surface tension therein causes the parts to self-align.However, passive alignment has not been employed for wafer-to-waferalignment. In particular, the high density of solder bumps required andthe thickness and mass of the wafer make such alignment impractical.

[0010] Another problem in integrating multiple optical elements formedon separate wafers at a wafer level arises due to the dicing process forforming the individual integrated elements. The dicing process is messydue to the use of a dicing slurry. When single wafers are diced, thesurfaces thereof may be cleaned to remove the dicing slurry. However,when the wafers are bonded together, the slurry enters the gap betweenthe wafers. Removing the slurry from the gap formed between the wafersis quite difficult.

[0011] Integrated elements are also sometimes made by injection molding.With injection molding, plastic elements can be made having two moldedelements located on opposite sides of a substrate. Multiple plasticelements can be made simultaneously with a multi-cavity injectionmolding tool.

[0012] Glass elements are also sometimes made by molding, as in U.S.Pat. No. 4,883,528 to Carpenter entitled “Apparatus for Molding GlassOptical Elements”. In this case, just as with plastic injection molding,multiple integrated elements are formed by molding two elements onopposite sides of a substrate. Glass molding however has disadvantagesof being expensive to make tooling and limited in size that can be used.

[0013] To make optics inexpensive, replication techniques are typicallyused. In addition to plastic injection molding and glass moldingdiscussed above, individual elements may also be embossed. An example ofsuch embossing may be found in U.S. Pat. No. 5,597,613 to Galarneauentitled “Scale-up Process for Replicating Large Area DiffractiveOptical Elements”. Replicated optics have not been used previouslytogether with solder self-alignment techniques. For each replicationmethod, many individual elements are generated as inexpensively aspossible.

[0014] Such replication processes have not been used on a wafer levelwith subsequent dicing. This is primarily due to the stresses imposed onthe embossed layer during dicing. When using embossing on a wafer level,unique problems, such as keeping the polymer which has been embossedsufficiently attached to the substrate, e.g., such that the alignment,especially critical on the small scale or when integrating more than oneelement, is not upset.

[0015] Further, these replication processes are not compatible with thewafer level photolithographic processes. In particular, replicationprocesses do not attain the required alignment accuracies forphotolithographic processing. Even if embossing was compatible withlithographic processing, it would be too expensive to patternlithographically on one element at a time. Further, the chemicalprocessing portion of lithographic processing would attack the embossingmaterial.

[0016] Other problems in embossing onto plastic, as is conventionallydone, and lithographic processing arise. In particular, the plastic isalso attacked by the chemicals used in lithographic processing. Plasticalso is too susceptible to warping due to thermal effects, which isdetrimental to the alignment required during lithographic processing.

SUMMARY OF THE INVENTION

[0017] Considering the foregoing background, it is an object of thepresent invention to efficiently produce integrated multiple opticalelements. Such efficient production is accomplished by forming theintegrated multiple optical elements on a wafer level.

[0018] It is further an object of the present invention to address theproblems arising when attempting to achieve such wafer level productionof integrated multiple optical elements. These problems include ensuringaccurate alignment, allowing precise dicing of the wafer to theconstituent integrated multiple optical elements when more than onewafer is bonded together, and providing additional features for allowingeasy incorporation of the integrated multiple optical element into anoverall system for a desired application.

[0019] It is another object of the present invention to provideembossing which has sufficient alignment for use with photolithographicfeatures and sufficient adhesion to withstand dicing.

[0020] These and other objects of the present invention will become morereadily apparent from the detailed description given hereinafter.However, it should be understood that the detailed description givesspecific examples, while indicating the preferred embodiments of thepresent invention, are given by way of illustration only, since variouschanges and modifications within the spirit and scope of the inventionwill become apparent to those skilled in the art from this detaileddescription.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not limitativeof the present invention and wherein:

[0022]FIG. 1 illustrates a first embodiment for bonding together twowafers;

[0023]FIG. 2 illustrates a second embodiment for bonding together twowafers;

[0024]FIG. 3a is a perspective view illustrating wafers to be bonded;

[0025]FIG. 3b is a top view illustrating an individual die on a wafer tobe bonded;

[0026]FIG. 4 illustrates a specific example of bonding two substratestogether;

[0027]FIG. 5 is a flow chart of the bonding process of the presentinvention;

[0028]FIG. 6 illustrates a surface to be embossed by a master element inwafer form;

[0029]FIG. 7 illustrates a wafer on which optical elements have beenformed on both sides; and

[0030]FIG. 8 is a cross-sectional view of a substrate having a hybridelement consisting of a microlens with a diffractive element integrateddirectly thereon.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] As can be seen in FIG. 1, a first substrate wafer 10 and a secondsubstrate wafer 12 are to be bonded together in order to provideintegrated multiple optical elements. A wafer is a disc, typically 4, 6,8, or 12 inches in diameter and typically having a thickness between 400microns and 6 mm.

[0032] These wafers have an array of respective optical elements formedthereon on either one or both surfaces thereof. The individual opticalelements may be either diffractive, refractive or a hybrid thereof.Dashed lines 8 indicate where the dicing is to occur on the wafers toprovide the individual integrated elements.

[0033] A bonding material 14 is placed at strategic locations on eithersubstrate in order to facilitate the attachment thereof. By surroundingthe optical elements which are to form the final integrated die, theadhesive 14 forms a seal between the wafers at these critical junctions.During dicing, the seal prevents dicing slurry from entering between theelements, which would result in contamination thereof. Since theelements remain bonded together, it is nearly impossible to remove anydicing slurry trapped therebetween. The dicing slurry presents even moreproblems when diffractive elements are being bonded, since thestructures of diffractive elements tend to trap the slurry.

[0034] Preferably, an adhesive or solder can be used as the bondingmaterial 14. Solder is preferable in many applications because it issmoother than adhesives and allows easier movement prior to bonding.Adhesives have the advantages of being less expensive for a number ofapplications, they can be bonded with or without heating, they do notsuffer with oxidation, and they can be transparent.

[0035] When using a fluid adhesive as the bonding material, theviscosity of the fluid adhesive is important. The adhesive cannot be toothin, or else it beads, providing indeterminant adhesion, allowing thedicing slurry to get in between the elements on the wafers, therebycontaminating the elements. The adhesive cannot be too thick, or therestoring force is too great and sufficient intimate contact between thesubstrates 10 and 12 to be bonded is not achieved. The fluid adhesivepreferably has a viscosity between 1,000 and 10,000 centipoise.Satisfactory epoxies include Norland 68 and Masterbond UV 15-7.

[0036] When a fluid adhesive is employed, it must be provided in acontrolled manner, such as ejected from a nozzle controlled inaccordance with the desired coordinates to receive the fluid adhesive.After alignment of the wafers, the entire assembly is cured, therebyhardening the fluid adhesive and completing the bonding.

[0037] When solder is used, an electroplating or sputtering process maybe employed. For example, a masking material may be put over thesubstrate wherever the substrate is not to have solder. Then the entirewafer is placed into a bath or sputtering chamber. Then solder is placedover the entire wafer and the masking material is pulled off, leavingsolder where there was no masking material. Once the wafers areappropriately aligned, the solder is then heated up to reflow. Thesolder is cooled and allowed to re-harden, thereby completing the bond.

[0038] When using the bonding material used alone as shown in FIG. 1 isa fluid adhesive, a more viscous adhesive is needed in order to ensurethat the bonding material remains where it is deposited. Even using aviscous adhesive, the adhesive still typically spreads over a relativelylarge area, resulting in a need for a larger dead space between elementsto be integrated to accommodate this spread without having the adhesiveinterfere with the elements themselves.

[0039] It is also difficult to control the height of the adhesive whenthe adhesive is used alone. This results in the amount of adhesive beingovercompensated and the height of the adhesive, and hence the separationbetween the wafers, often being greater than desired. The difficultycontrolling the height of the adhesive also results in air being trappedwithin the space containing the optical elements. This arises from theuncertainty as to the height and the timing of when a vacuum is pulledon the wafer pair. This air is undesirable, as it may expand uponheating and disrupt the bond of the elements.

[0040] Therefore, an advantageous alternative is shown in FIG. 2, inwhich only an individual integrated optical element of the wafer isshown. Stand offs 16 for each element to be integrated are etched orreplicated into the bottom substrate wafer 12 at the same time the arrayof optical elements are made for the substrate wafer 12, and typicallywill be of the same material as the substrate wafer. These stand offs 16preferably include a trench formed between two surfaces in which theadhesive 14 is to be placed. These trenches then provide precise spacingbetween the substrates to be bonded and provide more of a bondingsurface to which the adhesive 14 can adhere. This increased surface areaalso reduces beading problems.

[0041] When solder is used as the bonding material 14, solid stand-offsare preferably used to provide the desired separation between thewafers. The solder is then deposited in a thin, e.g., 4-5 micron, layeron top of the stand-offs. While the solder could be used alone as 'shownin FIG. 1, it is more feasible and economical to use the solder inconjunction with stand-offs.

[0042] The use of the stand-offs allows a more uniform and predictableheight to be obtained, resulting in less air being trapped between thebonded elements. A vacuum may now be pulled just before or at contactbetween the bonding material and the other substrate, due to thereduction in variability of the separation.

[0043] The substrate not containing the stand-offs may have notchesformed thereon to receive the stand-offs 16 therein. These notches canbe formed at the same time any optical elements on that surface areformed. In such a configuration, the stand-offs 16 and the correspondingnotches will serve as alignment features, facilitating alignment of thewafers to one another.

[0044]FIG. 3a shows the two substrates 10 and 12 prior to being bondedand diced. The individual optical elements 19 to be integrated mayconsist of one or more optical elements. Further, the optical elementson the wafers may be identical, or may differ from one another. Prior tojoining the wafers 10, 12, the bonding material 14 is placed on at leastone of the wafers in the manner described above. Advantageously, bothsubstrates 10 and 12 include fiducial marks 18 somewhere thereon, mostlikely at an outer edge thereof, to ensure alignment of the wafers sothat all the individual elements thereon are aligned simultaneously.Alternatively, the fiducial marks 18 may be used to create mechanicalalignment features 18′ on the wafers 11, 12. One or both of the fiducialmarks 18 and the alignment features 18′ may be used to align the wafers.

[0045]FIG. 3b shows a top view of a substrate 12 to be bonded includingthe location of the surrounding bonding material 14 for a particularelement 19. As can be seen from this top view, the bonding material 14is to completely surround the individual optical element, indicated at19.

[0046] For either embodiment shown in FIGS. 1 or 2, the bonding materialprovided either directly or using stand-offs completely seals eachelement to be individually utilized. Thus, when dicing a wafer in orderto perform the individual elements, dicing slurry used in the dicingprocess is prevented from contaminating the optical elements. Thus, inaddition to providing a structural component to maintain alignment andrigidity during dicing, the bonding material seal also makes the dicinga much cleaner process for the resultant integrated dies.

[0047] A specific example of integrated multiple optical elements isshown in FIG. 4. A refractive 20 is formed on a surface of the firstsubstrate 12. A diffractive 22 is formed on a surface of the othersubstrate 10. A diffractive 28 may also be formed on a bottom surface ofeither substrate. The stand offs 16 forming the trenches for receivingthe adhesive 14 are formed at the same time as a refractive lens.

[0048] When the lens 20 on the wafer 12 is directly opposite the otherwafer, the vertex of the lens 20 may also be used to provide theappropriate spacing between the substrates 10 and 12. If further spacingis required, the stand offs 16 may be made higher to achieve thisappropriate spacing.

[0049] In addition to using the fiducial marks 18 shown in FIG. 3a foralignment of the substrates 10, 12, the fiducial marks 18 may also beused to provide metalized pads 24 on opposite sides of the substratesrather than their bonding surfaces in order to facilitate alignment andinsertion of the integrated multiple optical element into its intendedend use. Such metal pads are particularly useful for mating theintegrated multiple optical elements with an active or electricalelement, such as in a laser for use in an optical head, a laser pointer,a detector, etc. Further, for blocking light, metal 26 may be placed onthe same surface as the diffractive 22 itself using the fiducial marks18.

[0050]FIG. 5 shows a flow chart of the general process of bondingtogether two wafers in accordance with the present invention. In step30, a substrate wafer is positioned relative to the bonding material tobe distributed. In step 32, the bonding material is applied to the waferin a pattern to provide sealing around the optical elements, eitherdirectly or with the stand-offs 16. In step 34, the second substratewafer is aligned with the first substrate wafer. Just before contact isachieved, a vacuum is pulled to remove air from between the substrates.In step 36, the wafers are brought into contact. In step 38, thealignment of the two wafers is confirmed. In step 40, the adhesive iscured or the solder is reflowed and then allowed to harden. Once firmlybonded, in step 42, the bonded wafers are diced into the individualelements.

[0051] The elements to be bonded together are preferably created bydirect photolithographic techniques, as set forth, for example, in U.S.Pat. No. 5,161,059 to Swanson, which is hereby incorporated byreference, for the diffractive optical elements, or in creating thespherical refractive elements by melting a photoresist as taught in O.Wada, “Ion-Beam Etching of InP and its Application to the Fabrication ofHigh Radiance InGAsP/InP Light Emitting Diodes,” General ElectricChemical Society, Solid State Science and Technology, Vol. 131, No. 10,October 1984, pages 2373-2380, or making refractive elements of anyshape employing photolithographic techniques used for making diffractiveoptical elements when the masks used therein are gray scale masks suchas high energy beam sensitive (HEBS) or absorptive gray scale masks,disclosed in provisional application Serial No. 60/041,042, filed onApr. 11, 1997, which is hereby incorporated by reference.

[0052] Alternatively, these photolithographic techniques may be used tomake a master element 48 in glass which in turn may then be used tostamp out the desired element on a wafer level in a layer of embossablematerial 50 onto a substrate 52 as shown in FIG. 6. The layer 50 ispreferably a polymer, while the substrate 52 is can be glass, e.g.,fused silica, or plastic, preferably polycarbonate or acrylic. Thepolymer is preferably a UV curable acrylate photopolymer having goodrelease from a master and good adherence to a substrate such that itdoes not crack after cure or release from the substrate during dicing.Suitable polymers include PHILIPS type 40029 Resin or GAFGARD 233.Dashed lines 58 indicate the dicing lines for forming an individualintegrated element from the wafer.

[0053] In the embodiment shown in FIG. 6, the layer of embossablematerial 50 is provided on the master element 48. A layer of adhesionpromoter 54 is preferably provided on the substrate 52 and/or a layer ofa release agent is provided on the master element 48 in between themaster element and the embossing material. The use of an adhesionpromoter and/or release agent is of particular importance when themaster and the substrate are of the same material or when the masternaturally has a higher affinity for adhesion to the embossable material.

[0054] The type of adhesion promoter used is a function of thephotopolymer to be used as the embossable material, the master materialand the substrate material. A suitable adhesion promoter for use with aglass substrate is HMDS (hexamethyl disilizane). This adhesion promoterencourages better bonding of the embossable material onto the substrate52, which is especially critical when embossing on the wafer level,since the embossed wafer is to undergo dicing as discussed below.

[0055] The provision of the embossable layer 50 on the master 48 and ofthe adhesion promoting layer 54 on the substrate 52 advantageouslyprovides smooth surfaces which are to be brought into contact for theembossing, making the elimination of air bubbles easier as noted below.The provision of the embossable layer on the master 48 also provides aconvenient mechanism for maintaining alignment of contacted, alignedwafer which have not been bonded, as discussed below.

[0056] If either the substrate or the master is made of plastic, it ispreferable to place the polymer on the other non-plastic component,since plastic absorbs strongly in the UV region used for activating thepolymer. Thus, if the UV radiation is required to pass through plastic,a higher intensity beam will be required for the desired effect, whichis clearly less efficient.

[0057] The use of embossing on the wafer level is of particular interestwhen further features are to be provided on the wafer using lithographicprocesses, i.e., material is selecting added to or removed from thewafer. Such further features may include anti-reflective coatings orother features, e.g. metalization pads for aligning the die diced fromthe substrate 52 in a system, on the embossed layer. Any such featuresmay also be lithographically provided on an opposite surface 56 of thesubstrate 52.

[0058] Typically an anti-reflective coating would be applied over theentire surface, rather than selectively. However, when using both ananti-reflective coating and metal pads, the metal would not adhere aswell where the coating is present and having the coating covering themetal is unsatisfactory. Further, if the wafer is to be bonded toanother wafer, the bonding material would not adhere to the surface ofhaving such an anti-reflective coating, thereby requiring the selectivepositioning of the coating.

[0059] For achieving the alignment needed for performing lithographicprocessing in conjunction with the embossing, fiducial marks as shown inFIG. 3 may be provided on both the substrate 52 and the master 48. Whenperforming lithographic processing, the alignment tolerances requiredthereby make glass more attractive for the substrate than plastic. Glasshas a lower coefficient of thermal expansion and glass is flatter thanplastic, i.e., it bows and warps less than plastic. These features areespecially critical when forming elements on a wafer level.

[0060] When placing the master on the substrate, the wafer cannot bebrought straight down into contact. This is because air bubbles whichadversely affect the embossed product would be present, with no way ofremoving them.

[0061] Therefore, in bringing the master into contact with thesubstrate, the master initially contacts just on one edge of thesubstrate and then is rotated to bring the wafer down into contact withthe substrate. This inclined contact allows the air bubbles present inthe embossable material to be pushed out of the side. Since the masteris transparent, the air bubbles can be visually observed, as can thesuccessful elimination thereof. As noted above, it is the presence ofthese air bubbles which make it advantageous for the surfaces to bebrought into contact be smooth, since the diffractive formed on thesurface of the master 48 could trap air bubbles even during suchinclined contact.

[0062] The degree of the inclination needed for removing the air bubblesdepends on the size and depth of the features being replicated. Theinclination should be large enough so that the largest features are nottouching the other wafer across the entire wafer on initial contact.

[0063] Alternatively, if the replica wafer is flexible, the replicawafer may be bowed to form a slightly convex surface. The master is thenbrought down in contact with the replica wafer in the center and thenthe replica wafer is released to complete contact over the entiresurface, thereby eliminating the air bubbles. Again, the amount of bowrequired is just enough such that the largest features are not touchingthe other wafer across the entire wafer on initial contact.

[0064] When using the fiducial marks themselves to align the masterelement 48 to the glass substrate 52 in accordance with the presentinvention, a conventional mask aligner may be used in a modifiedfashion. Typically in a mask aligner, a mask is brought into contactwith a plate and then a vacuum seals the mask and plate into alignment.However, a vacuum cannot be created when a liquid, such as a polymer,embossable material is on top of a wafer. Therefore, the above inclinedcontact is used. Once contact is established, the wafers are aligned toone another in a conventional fashion using the fiducial marks beforebeing cured.

[0065] Further, the intensity required to cure the polymer is very high,e.g., 3-5 W/cm², and needs to be applied all at once for a shortduration, e.g., less than 30 seconds. If enough energy and intensity arenot applied at this time, hardening of the polymer can never beachieved. This is due to the fact that the photoinitiators in thepolymer may be consumed by such incomplete exposure without fullpolymerization.

[0066] However, it is not easy to provide such a high intensity sourcewith the mask aligner. This is due both to the size and the temperatureof the high energy light source required. The heat from the high energysource will cause the mask aligner frame to warp as it is exposed tothermal variations. While the mask aligner could be thermallycompensated or could be adapted to operate at high temperatures, thefollowing solution is more economical and provides satisfactory results.

[0067] In addition to the inclined contact needed for placing the masterin full contact with the substrate in the mask aligner, once such fullcontact is achieved, rather than curing the entire surface, a deliverysystem, such as an optical fiber, supplies the radiation from a UVsource to the master-substrate in contact in the mask aligner. Thedelivery system only supplies UV radiation to individual spots on thepolymer.

[0068] The delivery system is small enough to fit in the mask alignerand does not dissipate sufficient heat to require redesign of the maskaligner. When using an optical fiber, these spots are approximately 2mm. Alternatively, a UV laser which is small and well contained, i.e.,does not impose significant thermal effects on the system, may be used.

[0069] The delivery system provides the radiation preferably to spots inthe periphery of the wafer in a symmetric fashion. For a 4 inch wafer,only about 6-12 spots are needed. If additional spots are desired forincreased stability, a few spots could be placed towards the center ofthe wafer. These spots are preferably placed in the periphery and aminimal number of these spots is preferably used since an area where atack spot is located does not achieve as uniform polymerization as theareas which have not been subjected to the spot radiation.

[0070] These tack spots tack the master in place with the substrate. Theillumination used for curing the tack spots is only applied locally andthere are few enough of these tack spots such that the area receivingthe illumination is small enough to significantly affect the rest of theembossable material. Once alignment has been achieved and the mastertacked into place, the substrate-master pair is removed from the alignerand then cured under the high intensity UV source over the entiresurface for full polymerization. The tack spots prevent shifting of thealignment achieved in the mask aligner, while allowing thesubstrate-master pair to be removed from the mask aligner to thereby usethe high energy light source external to the mask aligner for curing thepolymer.

[0071] Alternatively, the fiducial marks may be used to form mechanicalalignment features on the perimeter of the surfaces to be contacted. Themechanical alignment features may provide alignment along any axis, andthere may be more than one such mechanical alignment feature. Forexample, the stand-offs in FIG. 4 are for aligning the wafers along they axis, while the metal pads provide alignment of the wafer pair toadditional elements along the x and z axes. The alignment features arepreferably formed by the embossing itself.

[0072] The embossing and the lithographic processing on the oppositesurface may be performed in either order. If the embossing is performedfirst, it is advantageous to leave the master covering the embossedlayer until the subsequent processing on the opposite surface iscomplete. The master will then act as a seal for the embossed structure,protecting the polymer from solvents used during lithographic processingand keeping the features accurate throughout heating during lithographicprocessing.

[0073] If the lithographic processing is performed first, then moreprecise alignment is required during embossing to provide sufficientalignment to the photolithographic features than is required duringnormal embossing. Thus, embossing equipment is not set up to performsuch alignment. Then, the above alignment techniques are required duringembossing.

[0074] Once all desired processing has been completed, the wafer isdiced to form the individual elements. Such dicing places mechanicalstresses on the embossed wafer. Therefore, full polymerization andsufficient adhesion of the embossed portion to the substrate is ofparticular importance so that the embossed portion does not delaminateduring dicing. Therefore, care must be taken in selecting the particularpolymer, an adhesion promoter, and the substrate, and how these elementsinteract. Preferably, in order to avoid delamination of the embossedlayer during dicing, the adhesion of the polymer to the substrate shouldbe approximately 100 grams of shear strength on a finished die.

[0075] When both wafers to be bonded together as shown in FIGS. 1-4 havebeen embossed with a UV cured polymer, the typical preferred use of a UVepoxy for such bonding may no longer be the preferred option. This isbecause the UV cured polymer will still highly absorb in the UV region,rendering the available UV light to cure the epoxy extremely low, i.e.,in order to provide sufficient UV light to the epoxy to be cured, theintensity of the UV light needed is very high. Therefore, the use ofthermally cured resin to bond such wafers is sometimes preferred.

[0076] Alternatively, polymer on the portions not constituting theelements themselves may be removed, and then the UV epoxy could beemployed in these cleared areas which no longer contain the UV polymerto directly bond the glass substrate wafer having the UV polymer withanother wafer. A preferably way to remove the polymer includes providesa pattern of metal on the master. This metal blocks light, therebypreventing curing of the polymer in the pattern. When a liquid polymeris used, this uncured polymer may then be washed away. Other materials,such as the UV epoxy for wafer-to-wafer bonding or metal for activeelement attachment or light blocking, may now be placed where thepolymer has been removed.

[0077] In addition to the bonding of the two substrates shown in FIGS.1-4, the alignment marks may be used to produce optical elements on theother side of the substrate itself, at shown in FIG. 7. The creation mayalso occur by any of the methods noted above for creating opticalelements. The double sided element 70 in FIG. 7 has a diffractiveelement 72 on a first surface 70 a thereof and a refractive element 74on a second surface 70 b thereof, but any desired element may beprovided thereon. Again, metal pads 76 may be provided throughlithographic processing on the hybrid element.

[0078] A further configuration of an integrated multiple opticalelements is shown in FIG. 8, in which a diffractive element 82 is formeddirectly on a refractive element 84. The refractive element may be madeby any of the above noted photolithographic techniques. In the specificexample shown in FIG. 8, the refractive element is formed by placing acircular layer of photoresist 86 on a surface of optical material usinga mask. The photoresist is then partially flowed using controlled heatso that the photoresist assumes a partially spherical shape 87.Thereafter, the surface is etched and a refractive element 84 havingsubstantially the same shape as the photoresist 87 is formed by thevariable etch rate of the continually varying thickness of thephotoresist 87. The microlens 84 is then further processed to form thediffractive element 82 thereon. The diffractive element may be formed bylithographic processing or embossing.

[0079] The wafers being aligned and bonded or embossed may containarrays of the same elements or may contain different elements. Further,when alignment requirements permit, the wafers may be plastic ratherthan glass. The integrated elements which are preferred to bemanufactured on the wafer level in accordance with the present inventionare on the order of 100 microns to as large as several millimeters, andrequire alignment accuracies to ±1-2 microns, which can be achievedusing the fiducial marks and/or alignment features of the presentinvention.

[0080] When the optical elements are provided on opposite surfaces of asubstrate, rather than bonded facing one another, tolerable alignmentaccuracies are ±10 microns. This is due to the fact that when light istransmitted through the thickness of the glass, slight amounts of tiltcan be corrected or incorporated.

[0081] As an alternative to the fiducial marks used for passivealignment, the fiducial marks may be used to create mechanical alignmentfeatures, such as corresponding groves joined by a sphere, metalizationpads joined by a solder ball, and a bench with a corresponding recess.Only a few of these alignment features is needed to align an entirewafer.

[0082] All of the elements of the present invention are advantageouslyprovided with metalized pads for ease of incorporation, includingalignment, into a system, typically including active elements. Themetalized pads may efficiently be provided lithographically on the waferlevel.

[0083] The invention being thus described, it would be obvious that thesame may be varied in many ways. Such variations are not regarded as adeparture from the spirit and scope of the invention, and suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

What is claimed is:
 1. A method for forming an integrated multipleoptical element comprising: providing a bonding material surroundingeach first optical element in an array of first optical elements on afirst wafer; aligning a second wafer containing an array of secondoptical elements with said first wafer; treating the bonding material tothereby bond the aligned wafers; and dicing the bonded wafers, eachdiced, bonded wafer containing at least one optical element for each ofthe first and second wafers, thereby forming an integrated multipleoptical element.
 2. The method as recited in claim 1, wherein saidaligning step includes aligning fiducial features on each of thesubstrates to one another.
 3. The method as recited in claim 1, furthercomprising, prior to said bonding, lithographically manufacturing theoptical elements.
 4. The method as recited in claim 1, furthercomprising, prior to said bonding, replicating the optical elementsusing a master to emboss the optical elements into a polymer on a wafer.5. The method as recited in claim 1, further comprising, prior to saidaligning, precisely providing stand offs on said first wafer in order toensure an appropriate gap between said first and second wafers.
 6. Themethod as recited in claim 1, wherein the bonding material is UV curedepoxy and said bonding includes curing said epoxy.
 7. The method asrecited in claim 1, wherein the bonding material is solder and saidbonding includes reflowing and hardening the solder.
 8. The methodaccording to claim 1, wherein the adhesive provides sufficient sealingthat a dicing slurry applied during said dicing is prevented fromentering the gap between the substrates.
 9. The method as recited inclaim 1, further comprising providing metalized pads on a surfaceopposite said bonding surface for assisting and bonding an aligning themultiple optical element with active elements.
 10. The method as recitedin claim 1, further comprising providing stand-offs on said first wafer,said stand-offs surrounding each first optical element.
 11. The methodas recited in claim 10, further comprising providing solder on top ofsaid stand-offs.
 12. The method as recited in claim 10, furthercomprising providing a liquid adhesive in a channel formed by saidstand-offs.
 13. The method as recited in claim 1, further comprisingproviding alignment areas on each integrated element for bonding withactive elements.
 14. The method as recited in claim 1, furthercomprising providing fiducial marks on said first and second wafers. 15.A method of making optical elements on a wafer level comprising:lithographically making a master including an array of optical elements;embossing a replica of said array of optical elements by applying saidmaster to an embossable material; and dicing said replica to formindividual optical elements.
 16. The method as recited in claim 15,further comprising providing said embossable material in a thin film ona surface of the master prior to the embossing.
 17. The method asrecited in claim 16, further comprising providing an adhesion promoteron a glass substrate prior to the embossing.
 18. The method as recitedin claim 16, wherein said glass substrate contains fiducial marks andfurther comprising aligning said master to the fiducial marks.
 19. Themethod as recited in claim 15, further comprising coating said replicawith an anti-reflective coating.
 20. The method according to claim 15,further comprising selectively removing material from or adding materialto said replica in a predetermined pattern.
 21. The method according toclaim 20, wherein said selectively removing or adding occurs prior tosaid embossing.
 22. The method according to claim 20, wherein saidselectively removing or adding occurs after said embossing.
 23. Themethod according to claim 20, wherein said selectively removing oradding includes providing metal pads on a surface opposite a side ofsaid replica subject to said embossing.
 24. The method according toclaim 15, wherein said embossing includes embossing both sides of saidglass substrate.
 25. The method according to claim 24, wherein adifferent wafer master is used for embossing either side of said bothsides.
 26. The method according to claim 25, wherein a first wafermaster includes diffractive optical elements and a second wafer masterincludes refractive optical elements.
 27. The method according to claim15, further comprising providing fiducial marks on both said wafermaster and said replica.
 28. The method according to claim 15, furthercomprising confirming alignment of said replica and said wafer master ina mask aligner and tacking together said replica and wafer master oncealignment is confirmed.
 29. The method according to claim 28, furthercomprising removing said replica and said wafer master from the maskaligner after said tacking and curing the embossable material.
 30. Themethod according to claim 15, wherein said applying includes initiallybringing said wafer master into incomplete contact with said replica.31. An integrated dual sided multiple optical element comprising: asubstrate having two surfaces; lithographically defined optics on bothsurfaces; and additional lithographically defined features on at leastone surface from which material is selectively removed or added at onetime.
 32. The optical element according to claim 31, wherein one surfaceof said substrate includes a diffractive element for providing at leastone of beam splitting, creating multiple spots and diffuselyilluminating a specific area.
 33. The optical element according to claim32, wherein said diffractive element is a plurality of diffractiveelements.
 34. The optical element according to claim 31, wherein saidsubstrate is a wafer and said optics are an array of optical elements.35. The optical element according to claim 31, wherein said additionallithographically defined features include metal portions for blockinglight.
 36. The optical element according to claim 31 wherein saidadditional lithographically defined features include metal portions forassisting in bonding active element to the integrated multiple opticalelement.
 37. An optical element formed by the process recited inclaim
 1. 38. An optical element formed by the process recited in claim15.
 39. A hybrid optical element comprising a refractive optical elementand a diffractive pattern formed on a curved surface of said refractiveoptical element.
 40. The optical element as recited in claim 39, whereinsaid refractive optical element is formed lithographically.
 41. Themethod as recited in claim 15, wherein said master is a wafer.
 42. Themethod as recited in claim 20, wherein said selectively removing oradding is lithographic.
 43. The method as recited in claim 20, whereinsaid selectively removing or adding includes selectively removingembossable material.
 44. The method as recited in claim 43, wherein saidselectively removing embossable material includes providing metal in apattern on said master and, after said embossing, washing away uncuredembossable material.
 45. The method as recited in claim 43, wherein saidselectively removing or adding includes adding material where embossablematerial was removed.
 46. The method as recited in claim 28, whereinsaid tacking includes providing localized curing of said embossablematerial.
 47. The optical element as recited in claim 31, wherein opticson one surface are refractive and optics on another surface arediffractive.
 48. The optical element as recited in claim 31, whereinoptics on at least one of said two surfaces are formed by embossing. 49.The optical element as recited in claim 31, wherein lithographicallydefined optics include forming a master photolithographically andembossing optics using said master.
 50. The optical element as recitedin claim 31, further comprising features embossed from aphotolithographically created master.